Recently there has been substantial growth in integrated biorefineries in which any number of agricultural feedstocks can be converted into renewable fuels as well as higher-value chemicals, materials, and pharmaceuticals. A biorefinery operates on a concept similar to a petrochemical refinery. Incoming feedstocks for biorefineries include traditional agricultural products such as corn, milo, wheat, barley, millet, straw, sorghum, sugar cane, sugar beets, molasses, whey, fruits, and potatoes, and also other products that are currently classified as waste streams such as wood waste, bagasse, paper waste, and municipal solid waste. An appeal of such feedstocks is the carbohydrate content, which can be exploited as a reactant in a biorefinery. The products from a biorefinery may be intended for human consumption, such as sugar produced from sugar cane or molasses produced from sugar beets or for use as fuel or in chemical synthesis, such as ethanol and succinic acid produced from corn.
A particular application of biorefining is the production of fuel ethanol. As petroleum reserves become depleted and more expensive, the need for alternative, and preferably sustainable, energy sources increases. Ethanol is an option for partial or complete replacement of petroleum-based fuels for different applications. Ethanol-powered automobiles are a reality. Ethanol has advantages over the use of conventional gasoline as a renewable fuel source.
Currently both industrial ethanol (e.g., fuel) and beverage ethanol are produced on large scale from agricultural (natural) feedstocks by fermentation processes in which sugar is converted to ethanol and carbon dioxide by inoculant yeast. Many feedstocks can be used to provide the sugar for fermenting, including potentially, any starch or cellulosic material, which includes nearly all plants, as any starch or cellulose can be a precursor to sugar. Some of the common feedstocks particularly suitable for producing fuel ethanol include corn, milo, sorghum, sugar cane, sugar beets and molasses.
A significant problem facing biorefineries is spoilage of feedstocks during storage and transport. As raw agricultural materials, these feedstocks typically contain high levels of undesirable microorganisms such as bacteria, fungi, and undesirable yeasts that can degrade (spoil) the feedstock prior to entering a biorefining process. These microorganisms may be introduced as part of the original source of the feedstock or from preliminary preparation steps for corn feedstocks. Undesirable microorganisms may contain enzymes which convert the feedstocks into sugars that are metabolized by the microorganism facilitating its growth. Thus, valuable feedstock is lost as it is consumed by the microorganism. Growth of these microorganisms reduces the value of the incoming feedstock. In a particular example, a significant problem with the storage of molasses and sugar cane or sugar beet juice is deterioration of the sugar content due to the action of spoilage microorganisms such as Leuconostoc or Lactobacillus. 
While biocides are generally suitable to treat materials containing undesirable microorganisms, they are non-specific and attack target and non-target microorganisms. Biocides perform poorly in fermentation systems, because they can attack the inoculant yeast. Chlorine dioxide is a biocide that has been used in fermentation systems to treat microorganism infection. The chlorine dioxide may be introduced as chlorine dioxide gas from a suitable chlorine dioxide generator. Alternatively, stabilized chlorine dioxide (SCD) can be activated by contact with acid. Use of SCD to prevent microorganism infection in the presence of acid is disclosed in WO 2007/149450.
Ziegler discloses in WO 2007/097874 a method to reduce undesirable microorganisms, such as bacteria, contaminant yeast or killer yeast, in a fermentation process using chlorine dioxide (ClO2) gas. This process requires generation equipment and reactants necessary to generate ClO2 gas. The generated ClO2 must be used as it is produced, because it degrades when exposed to light, or when in contact with any organic matter such as would be present in a fermentation process. Ziegler teaches against use of stabilized chlorine dioxide as being difficult and imprecise, with potential to kill desire yeast and/or inhibit needed enzymes.
Stabilized chlorine dioxide is available commercially. Stabilized chlorine dioxide is generally a buffered solution (e.g., using a carbonate buffer for alkaline pH) of sodium chlorite, although other sources of chlorine dioxide also exist. Buffered sodium chlorite solutions are stable for long periods of time. Buffered sodium chlorite solutions can generate chlorine dioxide when activated, such as by chemical oxidation (e.g., with ozone or chlorine), electrochemical oxidation, or acidification (e.g., using a strong acid such as HCl). See, e.g., “Chlorine Oxygen Acids and Salts, Chlorous Acid, Chlorites and Chlorine Dioxide” by Jerry J. Kaczur and David W. Cawlfield, published online: 4 Dec., 2000, in Kirk-Othmer Encyclopedia of Chemical Technology.
There remains a need for a process to reduce the level of microorganism contaminants in feedstocks intended for use in a range of biorefinery processes. Carbohydrate-containing feedstocks, including sugar crops and cellulose feedstocks, are susceptible to spoilage from microorganisms. Biorefinery processes include manufacture of fuel ethanol, breakdown of cellulose containing biomass, sugar production (from sugar cane and/or sugar beets), sugar cane refining, processing of starches, such as potato starch and corn starch, among others. There is a further need for a process to prevent deterioration of feedstocks, such as carbohydrate solutions and suspensions, during storage and transportation.
It is desired to have a simple and economical process to stabilize carbohydrate feedstocks in storage and in transport. It is desired to have a process which does not introduce unnecessary and/or undesirable agents into the feedstock, particularly agents which adversely affect the quality of biorefinery products, such as ethanol.
The present invention meets these needs.